The present invention relates to a heat conductive circuit board for mounting various elements including semiconductor elements and resistor elements which generate a substantial amount of heat, and to a method for manufacturing such a circuit board.
With reference to FIG. 1 there is shown a known structure for a hybrid integrated circuit board. An insulating layer 6, provided on a metal substrate 3, is formed by dispersing fine particles of ceramic 5 in an organic polymer 2. The ceramic particles 5 have a high thermal conductivity and an excellent electrical insulation property. For example, alumina or boron nitride may be used as the ceramic particles. The organic polymer 2 may be formed of materials such as an epoxy resin, urethane resin and the like. The particles 5 are dispersed in the polymer 2 with a high concentration and uniformity.
In such a known circuit board the heat transfer of the organic polymer having the high concentration of heat conductive fine ceramic particles dispersed therein exhibits isotropy. Accordingly, in a hybrid integrated wiring board having a very thin (about 60 to 200 microns) insulating layer, the diffusion of heat generated by a circuit element has been heretofore controlled by the thermal conductivity in a thickness direction. For this reason, the use of highly heat conductive fine particles in an organic polymer has been a large barrier to obtaining a better hybrid integrated wiring board for diffusing heat generated by the semiconductor element or the like mounted on the wiring board.
More specifically, the insulating layer 6 is a mixture of organic polymer and ceramic. The organic polymer has strong bonding properties but a low thermal conductivity. The ceramic has excellent electrical insulation properties and a high thermal conductivity. Most of the heat generated by the circuit element is transferred to the substrate through the ceramic which has a thermal conductivity of roughly 50 to 300 times that of the organic polymer. Accordingly it is preferable to disperse the ceramic within the organic polymer so that the ceramic particles are arranged continuously as far as possible. For this purpose, the size and distribution of the constituent elements of the ceramic, and the filling rate thereof are carefully controlled. In particular, by dispersing highly heat conductive particulate ceramic and fibrous ceramic in a composite structure in the organic polymer, it is easy to obtain an insulating layer having a high filling rate as compared with the insulating layer in which only one type of ceramic is filled. A fibrous ceramic having a high thermal conductivity can be most easily obtained using alumina. When alumina fiber is made into a slurry aqueous solution with a binder dispersed therein and when the solution is subjected to a paper-making process, a sheet-type alumina paper is obtained. Then, a prepreg sheet which is a preliminary composite structure is obtained by using the alumina paper together with organic polymer containing the fine ceramic particles dispersed uniformly therein, and an insulating layer of a heat conductive circuit board is formed when the prepreg sheet is subjected to heating and compression molding while sandwiched between a conductor and a substrate. As a result in this insulating layer, the fibrous alumina the particulate alumina, and the ceramic such as boron nitride, etc., which are serving as filling materials, are dispersed uniformly both in a thickness direction (perpendicular to a planar surface) and in a surface direction (parallel to a planar surface) of the insulating layer.
In this connection, a laminated board having an alumina paper attached with a copper foil is disclosed in U.S. Pat. No. 4,578,308 (Japanese Laid-Open Patent Application No. 60-83831), and this laminated board is composed of a prepreg sheet and the copper foil affixed thereto. The prepreg sheet is formed of the alumina paper and a thermosetting resin. The alumina paper is formed by subjecting a mixture of alumina short fiber, as the principal component, and a microfibrillar organic fiber as a binding agent, to a paper making process. The alumina short fiber has a fiber diameter of roughly 100 microns or smaller and a fiber length at least about ten times greater than the fiber diameter.
The insulating layer is composed as described above. Since ceramic is dispersed uniformly in the organic polymer in a high concentration, it is difficult to provide a sufficient amount of organic polymer to insure bonding with a layer of conductive material (conductor layer). Accordingly, the conductor layer has a tendency to be deformed and adversely affected by stress load. For this reason, if a particular treatment as described, for example, in Japanese Laid-Open Patent Application No. 58-15290, is not conducted, there will be a large decrease in bonding strength, which can be represented by the peel strength defined in JIS (Japan Industry Standard), and the bonding strength will be limited to at most about 60 to 80%. Further, if an insulating layer containing ceramic bonded by using organic polymer is to be employed, the compounding ratio of the ceramic to the organic polymer must be optimized. Furthermore, on the one hand, to obtain a circuit board having a high thermal conductivity, it is necessary to use a large amount of ceramic. On the other hand, to obtain a circuit board including an insulating layer which has a high bonding strength, the content of organic polymer must be equal to or more than a predetermined amount, and the content of ceramic must be equal to or smaller than a predetermined amount. For example, a circuit board including an insulating layer in which the content of the ceramic is limited to roughly 60% or less by weight is disclosed in Japanese Utility Model Publication No. 46-25756. and 60 to 80% by weight in Japanese Laid-Open Patent Application No. 56-62388.
Further, a known method for manufacturing a circuit board of this type is illustrated in FIG. 2.
In FIG. 2, powdered ceramic is mixed with, for example, a liquid or a solution of heat-resistant resin such as epoxy resin, imide resin etc., as described in Japanese Laid-Open Patent Application No. 56-62388, and then the mixture is uniformly coated on a metal substrate. If a soluble resin is used after the resin has been coated on the metal substrate, the solvent must be removed or dried by use of hot air or a vacuum, or both. Then, a conductor is placed on the metal substrate, and the structure is subjected to heating and compression molding to thereby obtain a heat conductive circuit board. When highly heat conductive ceramic formed in a sheet is used, it is dipped in a liquid or a solution of heat-resistant resin, and a resin-impregnated sheet, for example, a prepreg sheet is produced. In this case, if the solution of the resin is used, similar to the above case, it is necessary to remove the solvent. The resin-impregnated sheet thus obtained is laminated with a metal substrate and a conductor, and a heat conductive circuit board is produced after heating and compression molding.
In the known heat conductive circuit board, on the one hand, when ceramic of the fine powder type is used in producing the insulating layer which is a composite structure of the resin and the ceramic, the mixture of the powdered ceramic and the resin must be coated on the substrate, and hence, the viscosity of the mixture is limited to a certain range. As a result, the composition of materials including the types of resins, the amount of solvent, the filling rate of the ceramic, and other parameters cannot be as freely selected as desired. On the other hand, when ceramic of the sheet-type is used, the resin must be impregnated in the ceramic sheet. The process of impregnating the resin in the ceramic sheet is inefficient for several reasons. First, the ceramic sheet is poor in strength and in flexibility. Second, as the obtained resin-impregnated ceramic sheet lacks sufficient smoothness, air bubbles are apt to remain in portions of the insulating layer which contact the metal substrate and the conductor during heating and compression molding. Third, in producing the mixture of the resin and the fine powder of ceramic, it has been necessary to mix these materials by using a roll or a vacuum mixing machine so that no air is rolled in, while adjusting the viscosity of the mixture and the filling rate of the fine powder. Fourth, a process of forming the resin-impregnated sheet, such as in a prepreg sheet, is required. In this process, to uniformly coat the resin in a fixed amount, techniques of removing excessive resin liquid and using two impregnation steps have been employed, while taking the relationship between the liquid concentration and the liquid viscosity into consideration. As described above, the formation of the insulating layer in a composite structure of the resin and the ceramic in the known method involves a problem in that since the ceramic and the resin are formed in a composite structure before laminating and compressing are performed, the productivity is low.